1. Field of the Invention
This invention relates to method and apparatus for inspecting printed circuit boards, and more particularly to method and apparatus for detecting defective patters on printed circuit boards.
2. Description of the Prior Art
Generally, in production of a printed circuit board, the creation of defective patterns such as cut-offs, short-circuits, irregular thinnesses of projections in the circuit printing step may result in the formation of a defectivce circuit board which causes serious damage to its circuit property if the circuit board is etched without removing those defective patterns therefrom. Nowadays, most circuit boards are inspected via the naked eyes of inspectors before or after the etching step. However, it is inevitable that "naked eye" inspections are occasionaly faulty or uneven because there are human factors such as individual differences between the inspectors, and or fatigue.
There is already in existance an apparatus for automatically inspecting printed circuit boards, whose inspection method can roughly be classified into a mutual comparison method and feature extraction method.
The two methods are generally employed in processing image data scanned by a video camera or CCD line sensor. In the mutual comparison method, defects in the circuit pattern are detected by the logical subtraction of the scanned bit image of the circuit board under inspection from the normal bit image stored in a memory device. On the other hand, in the feature extraction method, the defect is detected by extracting the local image data from the whole scanned image data and analyzing the feature such as the width, the area, or the angle of the pattern under inspection or analyzing whether the pattern have certain characteristics.
The mutual comparison method has the following drawbacks. The method requires such a large capacity memory device that it increases production costs of the inspection system. Although defects in the pattern can be detected without concern of its type or shape, the circuit board to be inspected must be aligned in a predetermined location for accurate inspection. Defective patterns of a smaller size than the order of the board alignment error cannot be detected, and the alignment error sometimes generates a pseudo defect.
On the contrary, the feature extraction method does not have such severe board alignment requirements and has a greater capability of detecting small defects than the mutual comparison method. It is, however, impossible for this method to detect defects which have a similar shape to the normal pattern and which are located at a position where the shape must not exist. However, in the detection of defects having large size according to this method, the image processing becomes complicated, thus requiring a sophisticated image processing system, so that it is very difficult to reduce the price of the apparatus.